[Colin2B (OP)]

...... If I have it straight, this is simply saying that a wave's speed is dependent on the properties of the medium in which it travels and not on the speed of the craft that caused it......

I've moved this from the other thread as it might get caught up in acrimonious posts.

As Alan says, "Light and sound are very, very different. You can't use one as an analogy of the other."  It's worth spending some time looking at this, so please excuse me if I labour some points which we've been over before.
Imagine a stationary loudspeaker. At low frequencies the cone moves quite slowly compared to the faster movement at higher frequencies, as the air is compressed by the movement it causes a sound wave which propagates at a speed defined by the properties of the medium, and for air between 20Hz-20kHz this speed will be constant relative to the medium.
Let us now move the loudspeaker towards you and imagine a point on the cone which is moving forward as the speaker moves forward, clearly the speed of that point (through the medium) will be that of the cone + the speed of the loudspeaker. However, if you think about it, that increase in speed is equivalent to an increase in frequency - the Doppler effect - even though the wave is still propagating through the air at the speed of sound (you can also think of the movement of the source towards you as causing the wavelength to compress, which means more wave crests/unit time, hence higher frequency). So in this case, the speed of sound relative to the medium is independent of the movement of the source, only the frequency changes.

Is this universally true for all media? Well it certainly isn't for all liquids and gases. With CO2, for example, the speed of sound varies with frequency, and if we look at the example of the moving loudspeaker we can see that a moving source results in increased frequency, hence in CO2 an increase in the speed of sound. So the speed of sound in CO2 depends on the movement of the source. In fact, because air contains a small amount of CO2, air also behaves in this way at frequencies above 28kHz.
So, what if we ignore this and take the special case of audible sound in air, surely this behaves in the same way as light, as suggested. This has been tested by looking at the Doppler effect of light emitted by Hydrogen ions traveling at high speed. They don't follow the classic Doppler  formula of sound in air, but need a relativistic correction eg Ives–Stilwell.

So, how do other analogies hold up? eg Galilean Relativity. Well, in SR it is important to consider the speed of light as measured by an observer in their reference frame, but as we have discussed in previous threads the speed of a wave in a medium, as measured by an observer, varies with the movement of the observer through the medium, or with the movement of the medium relative to the observer. As a reminder for any students: for water: "c=fλ , where c is traditionally used for the wave speed or "celerity". The term celerity means the speed of the progressing wave with respect to stationary water - so any current or other net water velocity would be added to it." http://hyperphysics.phy-astr.gsu.edu/hbase/waves/watwav2.html
" just as the speed of a ship would add to or subtract from the speed of ocean waves as measured from the ship"  http://www.britannica.com/science/relativity
For air, as any school physics student knows, "Two things affect the speed of sound in air
(i) the wind - if the wind is blowing in the same direction that the sound is travelling then the speed of sound is increased." http://www.schoolphysics.co.uk/age11-14/Sound/text/Speed_of_sound/index.html.
Although not often mentioned, the speed of sound measured by an observer moving through the medium also varies with the speed of the observer in the same way as " ...the speed of a ship would add to or subtract from the speed of ocean waves as measured from the ship". This can be easily visualised by considering a train passing through a station, and at the end of the platform is a whistle. To an observer on the platform, in still air, there is no change to the frequency of the whistle, but the observer putting a head out of the train window experiences a wind, hence a change in the observed speed of sound and a change of frequency. Under Galilean Relativity the observer moving through the medium and the medium moving past the observer are equivalent and results in a difference in the measured speed of sound compared to the stationary observer. It is important to note that the speed of sound relative to the medium has not changed. (Aside: I assume you have realised the the calculation - "Because the speed of sound is invariant in a medium it means that it does not obey Galilean transformations, thus the time taken for the sound of the siren to reach car (b) would be 1000/340 = 2.95 secs" - is wrong, and I'm sure you understand why).
There are other differences between the propagation of sound and light:
Because the speed of sound is affected by the movement of the medium, the propagation of sound in a moving medium, eg in a wind, is anisotropic.
There is also a difference with the Doppler effect. If for sound we consider the 2 cases of a stationary observer and a moving source, vs a stationary source and a moving observer, for the same speed of movement the 2 frequency shifts are different. This doesn't happen for light, the shift  only depends on the relative velocity of the source and observer. The reason for the difference is that the speed of sound is locked to the medium whereas the speed of the observer and the speed of light are not.
So, all in all we can see that Alan is right, we cannot draw analogies about the behaviour of light in a vacuum from the behaviour of sound in a medium.

[arcmetal]

Yes, I am afraid to say it, but this is a "straw man" argument.

[alancalverd]

If only. There are still fools around who think light consists of compression waves. Or possibly shear waves.

[Colin2B (OP)]

If only. There are still fools around who think light consists of compression waves. Or possibly shear waves.

You remind me that although I addressed arcmetal's assumption (that a wave's speed is dependent on the properties of the medium in which it travels and not on the speed of the craft that caused it) for sound in air, the assumption could cover any craft in any medium.
As you know, but clearly others don't, gravity waves travel at a speed determined by their wavelength - longer wavelengths travel faster. A boat with a displacement hull pushing its way through water creates a bow wave and the wavelength and speed of this wave are determined by the speed of the boat - the faster the boat, the longer the wavelength, the faster the wave propagates.
As you also know, but others might be interested, this has an limiting effect on the boat speed. When the wavelength equals the length of the boat there is a crest at the bow and another at the stern with the boat sitting in the trough. No matter how much force you use to drive the boat it cannot rise out of the trough (unless you have a planing hull) and so this is a limitation on the speed of displacement hulls, known as hull speed.

Like you I despair of people who claim great insights in physics, but make errors which would not be made by secondary school students.

[alancalverd]

Since the speed of sound in water is about 1500 m/s it's pretty obvious that the bow wave travels at the speed of the boat, not the speed of a free compression wave. And of course the shock wave of a supersonic aircraft travels faster than the speed of sound in air.

But however we generate and measure it, the speed of light in vacuo is always c, so it isn't a compression or shear wave in a medium.

[puppypower]

The intuition of an aether, stems from the assumption that our inertial reference is the zero state of the universe. This is a tradition that goes back to ancient times; earth is the center of the universe. This ancient tradition is supported by the theory there is no center of the universe and all reference is relative. Therefore the earth reference is as good as any other place to call the ground state.

From our earth reference ground state, we see all type of waves moving in various medium. We then use this same reference ground state to look at the universe.

The problem with this is, in our universe, matter net goes to energy or inertial references converts to speed of light reference. That is the net direction of potential in the our universe; toward the ground state C.

Once you use the C ground state, the speed of light is not moving relative to the C-ground state.  This is why there is no need for an aether. The aether is only needed if we assume an inertial ground state.

This debate about the aether is really about the paradox of reference choice. Those who assume no aether unknowing assume a C ground state, yet they perpetuate the math from the POV of an inertial round state. Those who believe in the aether, look at what the math does and not what the physicists say.

That math does not work if we assume a C ground state, but works well, if we assume the earth is the center of the universe; choice of reference. The aether is an aspect of the conversion factor, between the relative and absolute reference.

[jeffreyH]

You keep repeating that the ground state of the universe is the speed of light. Since a ground state is the lowest energy state this cannot be true. All velocities would have to tend naturally towards light speed which is not true. Please review classical mechanics and maybe quantum mechanics before continuing on this erroneous course.

[Colin2B (OP)]

Yes, I am afraid to say it, but this is a "straw man" argument.

I am somewhat confused by this statement.

My post was intended to correct a false assumption you were making:

...... If I have it straight, this is simply saying that a wave's speed is dependent on the properties of the medium in which it travels and not on the speed of the craft that caused it......

This was not your fault as the assumption was based on false information you were given on the behaviour of sound in air.

I am aware of 2 types of strawman argument:

1) to set up an argument, working hypothesis, outline plan or strategy etc that can then be argued 'beaten to death' to see is it stands up - this I have used a great deal. Clearly this is not the case here as all the information I gave is solid physics, well understood and accepted.

2) as a false argument to divert from the true question - this I don't use. Again not appropriate as the true question is the interpretation of the behaviour of waves in a medium and that was what I was addressing.

I assume you have by now worked out the correct answer to the 2 cars and siren example?
Have you also worked out the true response to the question of the 2 light beams

Quote from: PmbPhy on 08/05/2016 18:48:42

    Depending on what you mean you could be wrong. If you mean that if, in the inertial frame S, a beam of light was moving from along the x-axis in the positive direction with speed c and another beam was moving along the same x-axis in the negative direction with speed c then their relative speed of the two beams is 2c. What SR states is that that will be the same in any inertial frame of reference.

The OP clearly misunderstands PmbPhy and is confused over both frames of reference and Galilean Relativity as in the 2 cars example. In fact, I suspect he is somewhat confused over the basic understanding of speed judging by his response.

Normally I don't bother too much if people are getting their physics wrong, but in this case I could see it was leading you to false conclusions and that is unfair.
My only intention was to be helpful.

[arcmetal]

Yes, I am afraid to say it, but this is a "straw man" argument.

I am somewhat confused by this statement.

My post was intended to correct a false assumption you were making:

Upon reading your whole original post, it follows a classic definition of the straw man argument, which is that you win the argument by arguing against different concepts while sidestepping the original concept.

(I'd prefer to live by Occam's original razor: that if one wishes to understand a complex thing one should first remove the layers of that which is unnecessary.)

You list a set of points which is probably easier to talk about if I go one by one, which I can list in later posts.

I'll list those other points as time permits, my focus in these forums is to just pick up what I can, learn new things, etc., so that I can apply that to these things I am building.  If now and then I spot an easy question I can answer, I'll do so, to give something back.   I'm not so into debating theories of physics.  I am quite happy to let others believe or understand whatever they want, my only concern is in what I understand.

Normally I don't bother too much if people are getting their physics wrong, but in this case I could see it was leading you to false conclusions and that is unfair.
My only intention was to be helpful.

Thanks, that's much appreciated.

[Colin2B (OP)]

Upon reading your whole original post, it follows a classic definition of the straw man argument, which is that you win the argument by arguing against different concepts while sidestepping the original concept.

It would appear that we may have differing opinions about what was the original concept.
To me you were drawing the conclusion that "a wave's speed is dependent on the properties of the medium in which it travels and not on the speed of the craft that caused it" and you were basing that on the post you quoted which gave false information on the behaviour of sound in air. It also drew false conclusions about the behaviour of light in a medium based on the behaviour of sound. So the original concept I was discussing was the false information contained in that post, which is so full of misunderstandings that I felt it needed a full answer.

You & I have discussed elsewhere that there are a number of different media with different properties, if light does travel in a medium it is important that we understand what those properties are and not draw false conclusions based on analogies with inappropriate media. To do so takes us back to the pre M&M days and, as you pointed out, we need to move on from there or the concept of a medium will always be tainted by the misconceptions of that time.

.. I'm not so into debating theories of physics.  I am quite happy to let others believe or understand whatever they want, my only concern is in what I understand...

That's ok, but I'm sure you will agree that what we understand about physics has to be based on a firm understanding of the basic principles.
If you truly want to understand you will find many folks here who will be very happy to help.
Good luck in your journey.

[puppypower]

You keep repeating that the ground state of the universe is the speed of light. Since a ground state is the lowest energy state this cannot be true. All velocities would have to tend naturally towards light speed which is not true. Please review classical mechanics and maybe quantum mechanics before continuing on this erroneous course.

In our universe, there is an observed "net conversion" of matter to energy and not energy to matter. Do you agree? The most obvious way this occurs is through the forces of nature, such as the nuclear forces and EM forces; stellar fusion.

The reason objects don't just keep going faster and faster, toward the speed of light ground state, is explained by Special Relativity. To increase velocity, we will need to add force/energy, which then creates relativistic mass. The creation of relativistic mass causes the potential to go in the wrong direction; energy into mass. Before Einstein invented SR, what you said would have been true and kept us earth centric.   

There is an example of what you said. In the case of the electron and proton, the lowering of the EM force potential not only gives off energy at C, but the electrons will go faster as they get closer to the protons. It is like the figure skater pulling in their arms. In fact, one  could postulate that the formation of higher atoms in the periodic table is one way for electrons to move faster and faster toward the speed of light. As atoms build the inner electrons get closer to the larger charge of the nucleus. They can't get all the way to the C-level ground state, this way, due to special relativity. To go further, they will need to convert into energy.

There are many paths back to the C-ground state, many of which will compete. This competition gives the universe variety, and slows the return back to the ground state. For example, gravity causes space-time to contract in the direction of the point-instant; speed of light reference. This will cause pressure and temperature to increase on the matter, which can cause the EM forces of matter to gain potential away from C. Both paths are competing back to C with gravity winning out in the beginning. One common result will be a phase change of the matter, to allow the EM forces to cooperate with gravity toward C. For example, iron vapor and fluids becomes solid iron in the core of the earth.



I fully understand that few people have had time to practice visualizing our universe from the speed of light reference. We are all taught to use the earth as the ground state. The idea of the C ground state may seem backwards from that POV. Let me see if I can explain how light can be stationary, in the speed of light reference, yet light can also appear to move in inertial reference. 

If you were in a speed of light reference, the physical universe will appear to be like a point-instant. What they means is you can move from one end of the universe, to the other end, without having to move, since all the universe is superimposed onto a point. By just standing there, one is omnipresent relative to any inertial reference. The inertial references will see the universe more spread out. Even if a photon appears to be moving in any inertial reference, the photon never leave the stationary point in the C-reference. They don't move in the C ground state.

The question becomes how can one mathematically model being omnipresent, with respect to any inertial reference, to reflect the point universe one sees in the speed of light reference?  The simplest way is to assume that space-time becomes separated into time and space at the speed of light. As a visual, the fabric of space-time becomes unwoven, into separate threads of time and separated threads of space. As separate threads, one can follow a thread of space, independent of time.  One can be anywhere in zero time. Space-time coordinates time and space, placing mutual limits. But separate threads of time and space, are not limited by each other.

In the speed of light ground state, space-time breaks down into separated threads of time and separated threads of space. One can follow a time-line independent of space and/or a space-line independent of time. If you look at this in terms of entropy, fully separated and independent time and space threads gives endless variety; continuous reference.

To narrow down this infinite variety, so energy appears to have a fixed velocity and direction in space-time, we need to lower the entropy. This is what a quantum universe does. This can be done by pairing time and space threads in sequences, but not as a fabric.  As a visual, we line up a bunch of black threads and cross these with another layers of white threads. This looks like sort of like a fabric, but it is very unstable, and will fall apart if you move it; worm hole. A true space-time fabric is locked together and portable with time and space limiting each other. The not a fabric state of space-time (NAF), is consistent across all references, since it only occurs in the C reference, where space and time threads are independent.

The loss of entropy in C, defined by the NAF, is exothermic and is the potential from which inertial is spawned.

[jeffreyH]

In a universe where no particles had mass you may have a point. However in such a case interactions would depend upon trajectory. One particle following in the path of another could not interact. So the angle of approach would be critical. This is slightly reminiscent of gravitational interactions so that a model similar to yours may produce interesting ideas.

[arcmetal]

Upon reading your whole original post, it follows a classic definition of the straw man argument, which is that you win the argument by arguing against different concepts while sidestepping the original concept.

It would appear that we may have differing opinions about what was the original concept.

Yes, it would appear so.

To me you were drawing the conclusion that "a wave's speed is dependent on the properties of the medium in which it travels and not on the speed of the craft that caused it" and you were basing that on the post you quoted which gave false information on the behaviour of sound in air. It also drew false conclusions about the behaviour of light in a medium based on the behaviour of sound. So the original concept I was discussing was the false information contained in that post, which is so full of misunderstandings that I felt it needed a full answer.

I've reread McQueen's post in his thread in New Theories: "What is the speed of light really?",

McQueen says:

Yet coming back to the first example take car (a) travelling at 60 kmh is 1 km distant from car (b) travelling at 30kmh. Suppose that at this time car(a) activates a siren and also suppose that the speed of sound is  340m/s . At what time will the sound reach car(b) supposing that car(a) and car(b) continue to approach each other at the same speeds.  Would Galilean transformations apply and the combined speed with which car(a) and car(b) together with siren sound be 16.6m/s + 340m/s + 8.3m/s = 365m/s approx and they would meet at 1000/365 = 2.75 seconds. The answer is NO. Because the speed of sound is invariant in a medium it means that it does not obey Galilean transformations, thus the time taken for the sound of the siren to reach car (b) would be 1000/340 = 2.95 secs. During this time car (b) would have travelled 24.5 m approx.  This is a completely unexpected result, until one realizes that light also exhibits such invariance:

...and I find nothing odd with respect to his description of the speed of sound in a medium. He mentions other stuff about Galilean transformations, and about properties of the aether in that post, but as it pertains to the speed of sound in air his statements are rather of mundane things that everyone holds in common.... adding speeds of two approaching cars, speed of sound from a siren, nothing real spectacular there.

If you ignore his other statements in that post about the aether and such, that is fine, not everyone believes it, but as to the items describing sound waves traveling on air I find nothing out of the ordinary.  It was his simple idea that the speed of sound depends on nothing else but the properties of the medium and has no regard for the speed of the transmitters or receivers is what I found interesting.... so yeah, I am not seeing the: "false information on the behaviour of sound in air".

You & I have discussed elsewhere that there are a number of different media with different properties, if light does travel in a medium it is important that we understand what those properties are and not draw false conclusions based on analogies with inappropriate media. To do so takes us back to the pre M&M days and, as you pointed out, we need to move on from there or the concept of a medium will always be tainted by the misconceptions of that time.

Sorry for this tangent, but this brings up an interesting point.  Back in that time before the Einstein's SR, as you say, there were properties of the aether that people found to be nonsensical.  For example, that the aether should be a compressible medium, like a gas, but at the same time it needed to be an extremely stiff solid so that it could transmit transverse waves.   These incongruities are what people found to be displeasing, but I have been following the recent experiments with condensed matter, and I am finding it most interesting. 

There is the detection of transverse waves in superfluid He, thus it contains a property that everyone thought soley existed in solids.  Then there's the "anomalous isochoric compressibility" of solid He.  Meaning, they can compress the solid He without changing its volume, that is, its density is what changed... they can add more mass to the same volume thus compressing it. 

Imagine if the aether followed along these same lines...  For example, if mass attracts the aether, massive bodies would have a greater density of "aether". And a greater density of the aether means that light would be slower in that region, which is what we find when we send a light beam from air into a brick of glass. The light has a high speed within air, slower speed within the glass, then back to the fast speed again when it exits into the air.... Then assuming that the volume of this "aether" doesn't change since it occupies the entirety of the universe, its vessel, as a liguid would fill out its entire vessel. ... and so we get an "anomalous isochoric compressibility" property of the aether.

It is these intriguing parallels between the "aether" and the superfluids, supersolids that I find curious.

I can write about the speed of sound in C02, doppler effect, next time.

[Colin2B (OP)]

I've reread McQueen's post in his thread in New Theories: "What is the speed of light really?",

McQueen says:

Yet coming back to the first example take car (a) travelling at 60 kmh is 1 km distant from car (b) travelling at 30kmh. Suppose that at this time car(a) activates a siren and also suppose that the speed of sound is  340m/s . At what time will the sound reach car(b) supposing that car(a) and car(b) continue to approach each other at the same speeds.  Would Galilean transformations apply and the combined speed with which car(a) and car(b) together with siren sound be 16.6m/s + 340m/s + 8.3m/s = 365m/s approx and they would meet at 1000/365 = 2.75 seconds. The answer is NO. Because the speed of sound is invariant in a medium it means that it does not obey Galilean transformations, thus the time taken for the sound of the siren to reach car (b) would be 1000/340 = 2.95 secs. During this time car (b) would have travelled 24.5 m approx.  This is a completely unexpected result, until one realizes that light also exhibits such invariance:

...and I find nothing odd with respect to his description of the speed of sound in a medium. He mentions other stuff about Galilean transformations, and about properties of the aether in that post, but as it pertains to the speed of sound in air his statements are rather of mundane things that everyone holds in common.... adding speeds of two approaching cars, speed of sound from a siren, nothing real spectacular there.

If you ignore his other statements in that post about the aether and such, that is fine, not everyone believes it, but as to the items describing sound waves traveling on air I find nothing out of the ordinary.  It was his simple idea that the speed of sound depends on nothing else but the properties of the medium and has no regard for the speed of the transmitters or receivers is what I found interesting.... so yeah, I am not seeing the: "false information on the behaviour of sound in air".

If you are not seeing the false information about the behaviour of sound in air, then I see where the problem lies as you have not understood my post which addressed this issue.
It is possible that I pitched my reply at the wrong level and assumed a knowledge of physics. Let me retry.

Based on McQ's post you conclude "that a wave's speed is dependent on the properties of the medium in which it travels and not on the speed of the craft that caused it".
If this is true for some media it is not valid to assume it is true for all media. McQ is cherry picking his example and leaving the impression, as you conclude, that it is true for all media.
In the first part of my response I address this issue and show it is not true for all media. Due to lack of time I didn't include the example of gravity waves (reply #3) and Alan's additional comments in #4.

However, it is interesting to note your comment - "..as it pertains to the speed of sound in air his statements are rather of mundane things that everyone holds in common.... adding speeds of two approaching cars, speed of sound from a siren, nothing real spectacular there." - for it is here that McQ performs his own straw man argument:
"speed of sound is invariant in a medium it means that it does not obey Galilean transformations, thus the time taken for the sound of the siren to reach car (b) would be 1000/340 = 2.95 secs. During this time car (b) would have travelled 24.5 m approx.  This is a completely unexpected result, until one realizes that light also exhibits such invariance"
There are 2 problems with this, firstly the calculation will send most secondary school students into hysterical laughter, it is so unexpected a result simply because it is wrong. Secondly it performs a neat sleight of hand by misdirecting away from the true meaning of invariance.
The calculation: best understood by imagining still air where the sound from the siren is in the same frame of reference as the ground. When the siren starts the wavefront will travel relative to the ground at 340m/s, but car (b) will also be travelling towards it (through the medium) at 16.6m/s, so car and wavefront are moving at a closing speed of 356.6m/s. I'll leave you to work out the intercept time and distance, but you will see that from the reference frame of car (b) - the observer - Galilean transforms do apply and the speed of sound in air is not invariant.
It is on invariance that the second error is introduced and a sleight of hand to misdirect us away from the true meaning of invariance when applied to light (and sound). Let us be clear on one thing first, the speed of light is not a constant for all observers in all circumstances, and Einstein never said it was. What he said was (following the lead of Galilean Relativity) that the speed of light is constant for all observers in an inertial reference frame ie irrespective of their own speed relative to the light beam (and assuming that they are not accelerating nor in an equivalent situation to accelerating eg a gravitational field). Sorry to be so specific but it is important to be clear.
So going back to the situation of car (b) we can see that under this definition the speed of sound as observed by car (b) is not invariant, but follows Galilean Relativity. We have to compare like with like if we are to make comparisons.

Addressing these fallacious arguments was the whole reason for my opening post. I can see that your lack of physics background would have misled you into thinking that I was avoiding the actual question. This is not a problem and you will find that folks here are very understanding if you are a true seeker of knowledge. However, they do not suffer fools gladly and either McQ's is misrepresenting himself as an expert in this area or these are attempts to mislead, either is inexcusable. If someone claims to be an expert on history and then declares that the American Civil War took place in 1066 between the Spanish and Japanese you could expect them to either be ignored or ridiculed, but historians are unlikely to take them seriously.

I haven't had time to go through the second part of your post as it is important to establish mundane facts - or fundamentals as physicists call them.

If you wish to discuss sound in CO2 can I suggest you open a separate post. You might also want to read up on relaxation time of polyatomic fluids as background, and how molecule rotation affects the propagation of sound. I can't claim to be an expert but there are some good molecular chemists here who are.

[arcmetal]

The calculation: best understood by imagining still air where the sound from the siren is in the same frame of reference as the ground. When the siren starts the wavefront will travel relative to the ground at 340m/s, but car (b) will also be travelling towards it (through the medium) at 16.6m/s, so car and wavefront are moving at a closing speed of 356.6m/s. I'll leave you to work out the intercept time and distance, but you will see that from the reference frame of car (b) - the observer - Galilean transforms do apply and the speed of sound in air is not invariant.

Ah, so then you agree that the speed of sound is still 340 m/s in the medium, thus dependent on the medium properties.  Or, is it something else?
(I think you meant a closing speed of 323.4 m/s)

[Colin2B (OP)]

Ah, so then you agree that the speed of sound is still 340 m/s in the medium, thus dependent on the medium properties.

I have to ask whether you have actually read my posts??
If you do you will see that I not only make this clear in a number of places, but also take the trouble to explain why it is not always independent of the motion of the craft, and why it is not invariant of the motion of the observer - which is the same definition under which light is considered to be invariant.

(I think you meant a closing speed of 323.4 m/s)

How do you work that out?
Speed is the rate of change of distance. If a car and wavefront are moving towards each other the closing speed is the sum of their speeds.
323.4m/s would be the case if car (b) was moving away from the wavefront.

This might help you understand what is happening http://physics.ucsd.edu/students/courses/spring2009/physics2c/documents/2.3DopplerShift.notes.pdf

It will also make it clear that McQ is talking out of a place much darker than my hat []

Wise decision, the conclusions reached here are flawed
http://www.thenakedscientists.com/forum/index.php?topic=66803.0

Talking through your hat as usual...!

LOL, LOL

[arcmetal]

(I think you meant a closing speed of 323.4 m/s)

How do you work that out?
Speed is the rate of change of distance. If a car and wavefront are moving towards each other the closing speed is the sum of their speeds.
323.4m/s would be the case if car (b) was moving away from the wavefront.

Yes, if by closing speed you mean car (b) is heading into the wavefront, it adds up the speeds.... but moving away is still the same as "heading into", moving in "opposite directions", in which case you still add the speeds, and not subract --> so its not 323.4m/s.

We get (340-16.6) 323.4m/s if the car and wavefront are moving in the same direction.

[Colin2B (OP)]

Yes, if by closing speed you mean car (b) is heading into the wavefront, it adds up the speeds....

Yes, that is the correct use of the term closing. The items are closing the distance between them.
This is the scenario given in McQueen's incorrect calculation.

but moving away is still the same as "heading into", moving in "opposite directions", in which case you still add the speeds, and not subract --> so its not 323.4m/s.

This is usually termed opening or diverging as the 2 items will never meet - ie the items are not closing the distance between them.
Correct, again the speeds are added.

We get (340-16.6) 323.4m/s if the car and wavefront are moving in the same direction.

Correct. The wavefront will catch up with the car, but more slowly. This can also be termed closing, but is not the scenario given in McQueen's incorrect calculation.


Now you are in a position to complete the correct calculation of time to intercept and distance traveled by car (b). Nice little secondary school course work question.

We'll make a physicist of you yet []

[arcmetal]

We get (340-16.6) 323.4m/s if the car and wavefront are moving in the same direction.

Correct. The wavefront will catch up with the car, but more slowly. This can also be termed closing, but is not the scenario given in McQueen's incorrect calculation.


Now you are in a position to complete the correct calculation of time to intercept and distance traveled by car (b).

I see now that it is a great way of evading the issue, that is, the issue of a wavefront's speed depending only on the medium and not on its initiator.

[Colin2B (OP)]

I see now that it is a great way of evading the issue, that is, the issue of a wavefront's speed depending only on the medium and not on its initiator.

I think you need to go back and reread everything I've written - including Alan's posts.
Nothing has been evaded, we've been very clear and upfront. That's the reason I took the trouble to write a longer explanation than usual, rather than give you a glib answer.
There is no universal rule that a wavefront's speed depends only on the medium and not on the initiator, however, as we explained it does occur with some media in some situations. That's physics, try to understand it.
Or am I just wasting my time and you don't want to understand that there is no glib answer?